Protective means for electric circuits



Aug. 4, 1936. R. A. CUSHMAN ET AL 2,049,943

PROTECTIVE MEANS FOR ELECTRIC CIRCUITS Filed May 24, 1933 OUTPUT FILTER II l FIGS 40/ /22 /0 //2 //o I ill. /07 //3 fl /09 I16 I08] /06 /05 ll L//4 H, V35 w L i RACUSHMAN lNVENTORS LG YOUNG ATTO NEY Patented Aug. 4, 1936 PROTECTIVE MEANS FOR ELECTRIC CIRCUITS Robert A. Cushman, Morris Plains, and Lawrence G. Young, East Orange, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 24, 1933, Serial No. 672,614

21 Claims.

This invention relates to protective means for electrical circuits and more particularly to protective means for radio frequency transmission apparatus.

An object of this invention is to protect electrical apparatus from overloads of voltage.

Another object of this invention is the provision of means for the prevention of breakdown and damage to extremely expensive apparatus.

A more particular object is the provision of protective means, the operation of which causes no appreciable loss in the continuity of radio transmissions.

In accordance with a feature of this invention, means relieve the overload of potential in an electrical circuit for a predetermined period and subsequently restore the circuit to its operative condition.

In accordance with another feature of this invention a discharge path and automatic means associated with the output circuit of a radio frequency transmission system disables the input circuit for a fixed predetermined interval after which the apparatus again becomes operative.

In accordance with another feature of this invention means are provided whereby the high potential direct current for supplying the output circuits of a radio frequency amplifier is not discontinued during the operation of the protective mechanism.

Other features and advantages which are inherent in the invention will become apparent upon a consideration of the description given hereinafter and from a consideration of the appended claims.

In circuits of high power radio frequency transmitting systems especially high potentials are used. For transatlantic radio telephony, for example, a potential of over 18,000 volts is employed. It has been found that the potential in these circuits sometimes builds up to a point of which serious damage to the associated networks and apparatus results if it is not either removed or reduced. Heretofore, in order to protect the elaborate networks, a discharge path was provided in the output circuit. Whenever the dangerous radio frequency surges were produced, the currents would pass over the discharge path. However, when the surge was once produced, the normal radio frequency potential, although below that which caused serious damage to the apparatus, was sufficiently high to maintain the discharge. This condition resulted in the necessity of terminating the operation of the transmission equipment completely. Since time was an important factor in radio telephony or broadcasting, the situation not only severely hampered the continuity of the program or conversation, but in addition resulted in a serious economic loss. 5

In accordance with this invention means are provided whereby the expensive networks of radio frequency circuits are rendered immune from damage and the conversation or program is not appreciably interrupted by this protection.

A discharge path is provided in the circuit of a radio frequency transmitter at a point at which the excessively high potential radio frequency surges pass over the discharge path instead of 15 through the networks to be protected. Means associated with the discharge path disable the circuit for a fixed predetermined period in response to the discharge in the path and then restore the apparatus to its former operative condition. If the circuit is reenergized immediately after the discharge has been dispersed, the discharge path, due to the presence of ionized gas or vapor, usually breaks down again at a potential below that which causes damage to the apparatus. The short delay before reenergization permits the gas to dissipate.

A better understanding of this invention may be obtained by reference to the accompanying drawing in which:

Fig. 1 is a schematic of a generic form of the invention;

Fig. 2 is a schematic of the application of the invention to radio transmitting systems; and

Fig. 3 is a schematic of this invention applied to an oscillator.

Referring to Fig. 1, GEN represents a generator or source of power connected by means of a line I? to the apparatus or instrumentalities for which protection against excessive surges is desired. The generator comprises a field l9 and an exciting means l8 connected in series therewith by means of the line 3|. The exciting means |8 comprises another source of power such as a battery or another generator. A discharge path comprising the gap is connected between the ground 30 and one side of the line H at a point between the source of power GEN and the apparatus or load represented in the drawing by the rectangle labeled Load. Connected to the other side of the line l1 and ground 30 is another discharge path comprising the gap 2|. The gap 2| is connected to the line between the generator GEN and the apparatus or load. In series with each of the discharge paths and 2| are means for providing a relatively small potential difference which is sufficient to operate an actuating means without any appreciable darnage thereto upon the occurrence of a discharge in the respective paths. This means comprises two resistances 3 and 5 for the gap l and 23 and 25 for the gap 2i, resistance elements 3 and 23 having a considerably higher value than that of resistances 5 and 25, respectively. The lengths of the gaps l and 2t determine the potential at which a surge passes over the path. Connected in parallel to the resistance element 5 is an actuating means comprising an electromagnetic device l. VJhen a discharge passes over the path l, the potential across the low resistance element 5 is considerably less than that which exists across the high resistance 3 or the line ill. As a result, while the electromagnetic device is actuated, it is not damaged by the passage of current across the discharge path. A like actuating means comprising the electromagnetic device 2 1 and connected in parallel with the resistance element 255 is provided for the discharge path 2i. If the source of power is alternating current, means for converting that current into pulsating current may be inserted in series with the actuating means l and 26. The elements 2 and 22 comprising copper oxide rectifiers are media for serving this function. Other means, such as space discharge devices, may be employed. Means operated by the actuating means are provided which disables the circuit for a short interval after which the circuit is again restored to its normal condition. This means comprising an instrumentality responsive to the actuating means, circuit contacts, the instrumentality disabling the circuit by means of the circuit contacts, and a delay acting device for sustaining the disability for a fixed predetermined period. The instrumentalities responsive to the actuating means comprises an electromagnetic device 9, the circuit for the energization of which is completed through make contact I, battery 8 through the device 9 to armature B or make contact battery 8, through the device 9 to the armature 26. The circuit contacts for disabling the circuit comprises the break contact to and the armature l3. The opening of these contacts disables the field of the source GEN by opening the line 3i connecting the field it) to the exciting means it. The delay acting device compri es an auxiliary winding ll) of the electromagnetic device 9. Ihis device serves to sustain the energizaticn of that device for a predetermined interval depending upon the value of the variable resistance l l. The circuit for the auxiliary winding ill is completed through the engagement of make contact l4 and armature l2. Other delay acting devices such as those employing space discharge devices may be employed for this purpose.

The lengths of the discharge paths are regulated so that a surge passes over either path Whenever a potential exists on the line H which might damage the apparatus desired to be pro-- tected. When the surge occurs in gap l, a portion of the current passes through the device l, armature 6 engages contact ll, relay 5-? becomes energized, armature 82 engages with make contact i and armature l3 disengages from break contact E5. This latter action opens the line St. The field iii of the generator becomes deenergized and the surge ceases. A discharge over the gap 2!! energizes the device armature 2t engages with contact 2'27, relay 9 becomes energized, armature l3 disengages from contact l5, armature l2 engages contact it, the field of the generator becomes deenergized and the surge ceases. Although armature ii disengages with make contact ll upon the cessation of a surge over the discharge path l and armature 2t disengages from contact 2 in a like manner when the surge over the discharge path 2i ceases, relay 9 remains energized for a predetermined period thereafter due to the induced electromotive force stored in auxiliary winding is. This delay afiords a sufiicient time for the high potential to be dispersed from the system and the ionized gas to be dissipated from the gap. After the interval, electromagnetic device 9 becomes deenergized, armature i2 disengages from the contact I4, break contact again engages with armature E3, the circuit from the exciting source to the field is completed and the system returns to normal condition.

Fig. 2 shows the invention applied to a radio transmitting system, a line from the transmitter 853 passes through successively an audio frequency amplifier and a modulator 292 supplied with radio frequency currents by the oscillator 2st. The modulated currents then pass through an input filter and through a radio frequency amplifier 252 i. Since the anodes of the space discharge devices of the amplifier are supplied with a high potential direct current, a block condenser 58 is interposed between one side of the line and the output filter 265. The other side of the line from the radio frequency amplifier is connected to ground at. From the output filter the ungrounded side of the line pass-es to the antenna. radio transmitting system is of the conventional type well known to those skilled in the art.

For the protection of the apparatus from excessive surges, a discharge path 5!! is inserted across the line in the output circuit between the block condenser 58 and the output filter 205. In series with the discharge path are two resistance elements 53 and 55. Element 53 has a relatively high resistance and element 55 is low. A value of 2500 ohms has been found satisfactory for element 53, while that of 2 ohms is desirable for element 55. In parallel to the resistance 55 an electromagnetic device 5 3 is inserted. Means for the passage of pulsating current through the device 5 when a surge occurs in the path M is provided. This means comprises the copper oxide rectifier 52 in series with the device 55. A make contact till and an armature 55 are associated with the device 54, the energization of device 55 causing the engagement of the armature and make contact. For the protection of the apparatus from the high potentials resulting from atmosphere charges attracted by the antenna iii a similar discharge path It is inserted between the antenna and ground. In series with this discharge path are two resistance elements l3 and '55. An electromagnetic device It is connected across resistance l5. A copper oxide rectifier l2 connected in series with device l l serves to convert the radio frequency current to pulsating current. Associated with electromagnetic device it are make contacts I! and armature iii. An electromagnetic device 59 is energized by the engagement of the armature 56 with make contact 5? or the armature "it with make contact ll'l whenever their respectively associated electromagnetic devices M or l' l are energized. The circuits by which device 59 is energized are as follows: make contact '57, one side of line 8?. battery 58, electromagnetic device 59,

The above described the other side of line 81 to armature 56; or make contact 11, one side of line 81, battery 58, electromagnetic device 59, the other side of line 81 to armature 16. Connected across the line 61 between the input filter 203 and the radio frequency amplifier 204 is a line 66. Circuit contacts for closing that line comprise the armature 63 and the make contact associated with the electromagnetic device 59. The energization of the device 59 produces the disability of the transmitting circuit at a point between the input plter 203 and the radio frequency amplifier 204 by means of the closure of line 66 by the engagement of armature 63 with the make contact 65. A means for sustaining the disability of this circuit for a predetermined interval comprises an auxiliary winding 60 associated with the electromagnetic device 59, a variable resistance 6I connected in series with the auxiliary winding and a circuit completing means comprising the armature 62 and the make contact 64 associated with the device 59. By adjusting the variable resistance H, the time that the circuit remains disabled may be varied. When the relay 59 is energized, the engagement of armature 62 with the make contact 64 completes a circuit for the auxiliary winding 60 so that the device 59 remains energized for a short interval from the induced electromotive force produced in this winding. The block condenser 60 prevents the surge across the gap from being sustained by the direct current source of the radio frequency amplifier 204.

The lengths of the discharge paths 5I and II are adjusted for insuring protection to the apparatus at the minimum potential at which damage to the associated networks or apparatus of the system occurs. For the system employing 18,000 volts a length of .375 inch has been found satisfactory. When an excessive potential occurs, a surge passes over either paths 5| or II. The difference of potential across resistance elements 55 or I5 causes the passage of radio frequency currents through the respectively associated parallel circuits. The copper oxide rectifiers 52 and I2 convert this current into pulsating current. As a result either device 54 or I4 is actuated. Electromagnetic device 59 then becomes energized by the engagement of either armature 56 with the make contact 51 or armature I6 with make contact 11 depending upon whether device 54 or I4 is energized. As a result of the energization of electromagnetic device 59, armatures 62 and 63 engage respectively with make contacts 64 and 65. The closure of line 66 disables the input circuit of the radio transmitter system with the consequential relieving of the high potential in the output circuit. The engagement of the make contact 64 with the armature 62 completes the circuit for the energization of the relay 59 from the induced electromotive force produced in the auxiliary winding. Since a lesser potential is required to maintain the surge after the initial discharge, this short interval of continued disability of the input circuit insures the removal of the potential sufficient to maintain the surge. In addition the ionized gases resulting from the surge are dissipated. It has been found that threetenths of a second is usually satisfactory for this purpose. While the potential of the output circuit of the radio frequency amplifier of certain transatlantic radio systems is approximately 18,000 volts, that of the input of these same systems is less than 10 volts. After the interval ,of delay device 59 becomes deenergized, contact 65 is disengaged from armature 63 and the circuit again becomes operative.

In Fig. 3 the application of the invention to an oscillator is illustrated. The numerical designation I28 represents a space discharge device comprising a grid, cathode and anode. The elements I24 and I 25 are variable condensers connected in series with each other and respectively in the input and output circuits of the oscillator. Connected across the condensers is an inductance I26. By adjusting the condensers I24 and I25 the oscillator may be tuned to the desired frequency. In shunt with the output circuit is a choke coil I23 in series with a source of current I22 for the anode of the space discharge device. The function of the choke coil I22 is the prevention of an excessive amount of current from being shunted around the condenser I 25. A condenser I29 is in series in the input circuit. Between the condenser I29 and the grid of the space discharge device, a choke coil I30 is shunted across the input circuit. The oscillator, the elements of which are outlined above is well known and is a modification of the oscillation generator fully described in U. S. Patent 1,624,537 granted to E. H. Colpitts on April 12, 1927.

Across the output circuit between the condenser I3I and the ground I32 is a discharge path comprising the gap IOI. In series with the discharge path are means for providing a relatively small potential difference to energize an actuating means. The means for providing the potential difference comprise elements I03 and I05 connected in series with the gap IOI, element I03 having a considerably higher resistance than element I05. The actuating means comprising the electromagnetic device I04 is connected across the element I05. Means comprising the copper oxide rectifier I02 for rendering the actuating means responsive to the alternating currents passing through the parallel circuit are inserted in series with the electromagnetic device I04. An electromagnetic device I09 responsive to the actuating means is energized by the completion of the circuit: armature I06, make contact I0I, battery I08 to device I09. Means for sustaining the energization of the device I09 for a predetermined interval after its initial energization comprises an auxiliary winding IIO, the circuit for which is completed through the variable resistance III, the armature II2, make contact II4 to the winding IIO. By adjusting the resistance III, the interval may be varied. A circuit for disabling the input circuit of the oscillator comprises a line I35 connected across the input circuit. The circuit is completed by the engagement of armature H3 and make contact II5 of electromagnetic device I09. A block condenser I3I in the output circuit between the source of current I22 and the discharge path prevents the potential from the direct current source I22 from continuing the surge after the disability of the input circuit.

The length of the path IOI is adjusted so that a surge passes over the gap whenever an excessive potential exists in the output circuit. When a surge passes over the gap I0 I, a small difference of potential is produced across the element I 05. Alternating current starts to flow through the parallel circuit comprising the electromagnetic device I04. By means of the rectifier I02, the alternating current is converted for the energization of the electromagnetic device I04. Make contact Hl'l engages with the armature I06. Electromagnetic device Hi9 becomes energized as a result of this action. Armatures H3 and H2 engage respectively with make contacts H5 and H4. The input circuit of the oscillator becomes disabled. As a consequence, the potential of the output circuit is materially reduced. The surge ceases and make contact It! is disengaged from the armature Hi6. However, electromagnetic device lfl9 remains energized for a short period after this disengagement by virtue of the induced electromotive force stored in the auxiliary winding l l 0. The disability of the input circuit therefore continues for a short period after the excessive potential appears in the output circuit. After this period has passed, armatures H3 and H2 disengage respectively from make contacts H5 and H4 and the oscillator again becomes operative.

While the protective device is described as associated with a particular oscillator, it is equally applicable to all types of oscillators.

What is claimed is:

1. In combination, an electric circuit, a protective system comprising a discharge path in which the discharge is sustained at a potential lower than that required to initiate discharge and means for disabling said circuit for a fixed predetermined period in response to the occurrence of a discharge in said path, said period being sufficient to prevent the occurrence of a discharge in said path at a lower potential than that initially required.

2. In combination, an electric circuit, a protective system comprising a discharge path in which partial ionization of the gases contained therein is produced upon the occurrence of a discharge, means for disabling said circuit in response to the occurrence of a discharge in said path, and means for restoring said circuit to its operative condition after a fixed predetermined period sufiicient for the ionized gases produced by said discharge in said path to be dissipated.

3. In combination, an electric circuit, a protective system comprising a discharge path in which partial ionization of the gases contained therein is produced upon the occurrence of a. discharge, means for providing a relatively small potential upon the occurrence of a discharge in said path, actuating means responsive to said potential for disabling said circuit, and means for restoring said circuit to its operative condition after a fixed predetermined period sufiicient for the ionized gases produced by said discharge in said path to be dissipated.

4. In combination, an electric circuit having a generator comprising an armature and field, a protective system comprising a discharge path in which partial ionization of the gases contained therein is produced upon the occurrence of a discharge, means for disabling the field of said generator on the occurrence of a discharge in said path, and means for restoring said field to its operative condition after a fixed predetermined period sufiicient for the ionized ases produced by said discharge in said path to be dissipated.

5. In combination, an electric circuit, a protective system comprising a discharge path in which partial ionization of the gases contained therein is produced upon the occurrence of a discharge, means for providing a relatively small potential upon the occurrence of a discharge in said path, actuating means responsive to said potential, means operated by said actuating means for disabling said circuit, and means for restoring said circuit to its operative condition after a fixed predetermined period sufilcient for the ionized gases produced by said discharge in said path to be dissipated.

6. In combination, an amplifying circuit comprising input and output circuits, a discharge pathin said output circuit, and means responsive to any discharge in said path for disabling said input circuit.

7. In an amplifying circuit comprising input and output circuits, a protective device comprising a discharge path in said output circuit, means to disable the input circuit for a fixed predetermined period upon the occurrence of any discharge in said path, and means for restoring said circuit to its operative condition after said period.

8. In an amplifying circuit having an output circuit connected thereto, a protective device comprising a discharge path in said output circuit, said discharge path characterized in this that partial ionization of the gases contained therein is produced upon the occurrence of a discharge, and means to disable said amplifying circuit upon the occurrence of a discharge in said path, and means for restoring said circuit to its operative condition after a fixed predetermined period sufficient to permit the ionized gases produced by said discharge in said path to be dissipated.

9. In an amplifying device comprising input and output circuits, a protective system comprising a discharge path in said output circuit, electromagnetic means responsive to any discharge in said path for disabling said input circuit, and means for sustaining the disability of said input circuit for a fixed predetermined interval.

10. In an amplifying circuit comprising input and output circuits, a protective system comprising a discharge path in said output circuit, means for providing a relatively small electromotive force upon the occurrence of any discharge in said path, means responsive to said electromotive force for disabling said input circuit and means for sustaining the disability for a fixed predetermined interval.

11. In an amplifying circuit comprising input and output circuits, a protective system comprising a gaseous discharge path in said output circuit, means for providing a relatively small electromotive force upon the occurrence of a discharge in said path, an electromagnetic device responsive to said electromotive force, circuit contacts for disabling said input circuit, means actuated by said electromagnetic device for controlling said circuit contacts to disable said input circuit, and means for sustaining the disability of said input circuit for a fixed predetermined period sufficient to permit the ionized gases produced by said discharge in said path to be dissipated.

12. In an oscillating circuit comprising input and output circuits, a discharge path in said output circuit, means responsive to any discharge in said pathfor disabling said input circuit for a fixed predetermined interval and means for restoring said input circuit to its operative condition after said interval.

13. In an amplifying circuit comprising input and output circuits, a protective system comprising means for short-circuiting said input circuit when an excessive potential exists in said output circuit.

14. In combination, a radio frequency amplifier, input and output circuits connected thereto, a filter connected to said output circuit and a protective system comprising means for shortcircuiting said input circuit of said radio frequency amplifier when an excessive potential is produced in said output circuit between said radio frequency amplifier and said filter.

15. In a radio transmitter comprising a radio frequency amplifier having an output circuit connected thereto, a filter connected to said output circuit and an antenna connected to said filter and a protective system comprising means for disabling said radio frequency amplifier when an excessive potential is produced between said filter and said antenna.

16. In a radio transmitting system comprising a radio frequency amplifier having input and output circuits, a filter connected to said output circuit, a protective system comprising a discharge path in said output circuit between said radio frequency amplifier and said filter, and means for disabling the input circuit of said amplifier upon the occurrence of any discharge in said path.

1'7. In an alternating current circuit a protective system comprising a discharge path, in which partial ionization of the gases contained therein is produced upon the occurrence of a discharge, means for providing a relatively small electromotive force upon the occurrence of a discharge in said path, circuit contacts for disabling said circuit, rectifying means to convert the alternating current provided by said electromotive force to unidirectional current, actuating means responsive to said unidirectional current for controlling said circuit contacts and a delayacting device for sustaining the disability of said circuit for a fixed predetermined period sufiicient to permit the ionized gases produced by said discharge in said path to be dissipated.

18. An amplifying system comprising a plurality of stages connected in cascade, each stage comprising a space discharge device, and a protective system comprising means for short-circuiting an earlier stage of said system when an excessive potential exists in a later stage.

19. An amplifying circuit comprising input and output circuits, a space discharge device connected to said input circuit and a second space discharge device connected in cascade to said first space discharge device and connected to said output circuit and means for short-circuiting said input circuit when an excessive potential exists in said output circuit.

20. In an amplifying system comprising input and output circuits, a protective system comprising means responsive to an excessive potential in said output circuit for disabling said input circuit for a fixed predetermined period and for restoring said input circuit to its operative condition after said period.

21. In an amplifying circuit having input and output circuits connected thereto, a protective system comprising means, responsive to an excessive potential in said output circuit, for removing the potential from said input circuit.

ROBERT A. CUSHMAN. LAWRENCE G. YOUNG. 

